Transient suppression circuit for d. c. motor drive system

ABSTRACT

A control circuit for a d.c. system including a transient suppression network connected in parallel with an inductive load and comprising a plurality of series connected, like poled, unidirectional current conducting elements, such as diodes, and an inductive impedance connected in parallel with at least one of the unidirectional current conductive elements.

United States Patent Ehret [451 Oct. 10, 1972 TRANSIENT SUPPRESSIONCIRCUIT FOR D. C. MOTOR DRIVE SYSTEM Robert J. Ehret, Los Altos, Calif.

Assignee: Beckman Instruments, Inc.

Filed: Jan. 27, 1971 Appl. No.: 110,011

Inventor:

US. Cl. ..317/l48.5 B, 318/492 Int. Cl. ..I I0lh 47/32 Field of Search..3l7/DIG. 6, 11 E, 148.5 B;

References Cited UNITED STATES PATENTS Corey 0321/47 3,325,715 6/1967.lacoby ..3l8/492 Primary Examiner-L. T. Hix Att0rneyRobert J.Steinmeyer and James M. Thomson [ ABSTRACT A control circuit for a dcsystem including a transient suppression network connected in parallelwith an inductive load and comprising a plurality of series connected,like poled, unidirectional current conducting elements, such as diodes,and an inductive impedance connected in parallel with at least one ofthe unidirectional current conductive elements.

4 Claims, 1 Drawing Figure UNIJUNCTION TRIGGER CIRCUIT PATENTEDucI 10I972 umauucnou TRIGGER CIRCUIT INVENTOR ROBERT J. EHRET BY f/ZJMMWTRANSIENT SUPPRESSION CIRCUIT FOR D. C. MOTOR DRIVE SYSTEM BACKGROUND OFTHE INVENTION l. Field of the Invention This invention relates ingeneral to control circuits for d.c. systems having an inductive load,such as a d.c. motor drive system, and more particularly to a signaltransient suppression network for suppressing line transients arisingfrom the gating of switching elements, such as silicon controlledrectifiers.

2. Description of the Prior Art Present drive circuitry employed with ad.c. motor conventionally utilizes a pair of silicon controlledrectifiers (SCRs) in conjunction with a full wave bridge rectifyingnetwork with the SCRs being triggered into conduction on alternate halfcycles of an a.c. line voltage input to provide a d.c. energizing signalto the motor. Triggering pulses for the SCRs are generally derived froma unijunction trigger circuit whose triggering time is dictated andcontrolled by a signal which is the function of the desired motor speed.In this manner the speed of the motor may be accurately controlled bymerely varying the firing angle of the silicon controlled rectifiers.

It has been found that the gating of silicon controlled rectifiersutilized in such an environment may result in large line voltage andcurrent transients. Such transients result in excessive powerdissipation and may, and ofttimes do, lead to voltage breakdown anddestruction of various circuit components.

SUMMARY The present invention contemplates a transient suppressionnetwork for reducing large voltage and current transients which occurwhen gating switching elements, such as silicon controlled rectifiers,associated with a full wave rectifier network in a d.c. system having aninductive load. To this end there is provided a transient suppressionnetwork connected in parallel with the inductive load and comprising aplurality of series connected, like poled, unidirectional currentconducting elements, such as diodes, and an inductive impedanceconnected in parallel with at least one of the unidirectional currentconductive elements.

Accordingly, the primary object of the present invention is theprovision of a transient suppression network in a d.c. circuit having aninductive load for reducing voltage and current transients caused by thegating of switching elements, such as silicon controlled rectifiers.

This and other objects and other advantages of the invention will becomeapparent from the following detailed description read in conjunctionwith the accompanying drawing in which:

DETAILED DESCRIPTION OF THE DRAWING The sole FIGURE is a schematicdiagram, partially in block form, of the present transient suppressionnetwork included in a control circuit for a series wound d.c. motor.

At the outset it should be emphasized that while for descriptivepurposes the invention is illustrated and discussed in connection with ad.c. motor control system, it is contemplated that the invention may beemployed as well in other d.c. systems having an inductive load andnothing in the following description should be construed to limit theinvention to d.c. motor control systems.

Referring now to the drawing, it will be observed that the referencenumeral 1 designates a source of a.c. line voltage which is adapted tobe connected to input terminals 2 and 3 for providing energizing powerto series wound d.c. drive motor 4. The a.c. energizing signal iscoupled to d.c. motor 4 by way of a full wave bridge rectifier 5 andconnecting line 6. Full wave bridge rectifier 5 includes two oppositelypoled diodes 6 and 7, respectively, and two oppositely poled siliconcontrolled rectifiers (SCRs) 8 and 9, respectively.

Rectifier 5, in a manner well known in the art, rectifies the a.c. inputsignal waveform from a.c. source 1 to provide a d.c. energizing currentwhich is applied to d.c. drive motor 4. SCRs 8 and 9 serve as signalgating means to vary the magnitude of the d.c. energizing currentprovided by rectifier 5. The firing angle of the SCRs 8 and 9 iscontrolled by pulses supplied from a unijunction trigger circuit 10which pulses are impressed upon the gating electrode of each SCR 8 and 9via the transformer 11 having a primary winding 12 connected to thetrigger circuit 10 and a pair of secondary windings 13 and 14 connectedbetween the gating and cathode electrodes of SCRs 8 and 9, respectively.By increasing or decreasing the firing angles of SCRs 8 and 9, themagnitude of the energizing current delivered to d.c. motor 4 may beaccurately varied and the motor speed thereby controlled. A typicalunijunction trigger circuit is shown and described in the GeneralElectric SCR Manual, Section 8.6, FIG. 8,25, dated 1964.

Connecting line 16 serves to interconnect series connected field winding15 and armature winding 17 of d.c. drive motor 4 in series with fullwave bridge rectifier 5. An inductor 18 is also connected in a serieswith the field and armature windings l5 and 17 to assist in thereduction of the ripple of the d.c. energizing current.

A transient suppression network, designated generally by the referencenumeral 19, is connected in parallel with the series connected field andarmature windings l5 and 17, respectively, of d.c. drive motor 4.Transient suppression network 19 comprises a pair of serially connected,like poled, unidirectional current conducting elements 20 and 21respectively, and an inductive impedance 22, consisting of an inductor,connected in parallel with unidirectional current conducting element 20.While in the illustrated embodiment unidirectional current conductingelements 20 and 21 comprise diodes, it will be appreciated by thoseskilled in the art that other appropriate unidirectional currentconducting elements may be utilized. It is significant to note thatdiodes 20 and 21 are poled to be rendered conductive by the back e.m.f.generated in the armature winding when neither SCR 8 or 9 is gated on.

Finally, a series connected resistor 23 and capacitor 24 is connected inparallel with armature and field windings l7 and 15, respectively, ofd.c. motor 4 to assist in the reduction of any voltage transients due todistributed power line inductance.

In operation the a.c. signal from source 1 is impressed across the inputterminals of full wave bridge rectifier 5. On the positive half cycle ofthe a.c. signal d.c. current flows through diode 6, connecting line 16,field winding 15 and armature winding 17 of d.c. motor 4, inductor l8,and silicon controlled rectifier 9 to input terminal 3. On the negativehalf cycle of the a.c. signal, d.c. current flows from input terminal 3through silicon controlled rectifier 8, connecting line 16, field andarmature windings 15 and 17, respectively, of d.c. motor 4, inductor 18,and diode 7 to input terminal 2. It will be noted that both siliconcontrolled rectifiers 8 and 9 are simultaneously triggered intoconduction by trigger pulses derived from unijunction trigger circuit10. However, due to their oppositely poled arrangement, SCR 9 passes thepositive half cycle while SCR 8 passes the negative half cycle of thea.c. signal input.

As previously mentioned, the magnitude of the d.c. energizing signalsupplied to d.c. motor 4 is controlled by way of unijunction triggercircuit 10. That is, by varying the times SCRs 8 and 9 are gated openand closed the magnitude of the current flowing during each half cycleof the a.c. signal may be accurately determined thereby controlling thespeed of d.c. motor 4. In practice a short time interval exists duringwhich both SCRs 8 and 9 are gated off thus, opening the circuit betweeninput terminals 2 and 3. Under this condition the current produced bythe collapsing fields associated with the armature and field windings l5and 17 is sufficient in magnitude to fiow through diodes 7 and 6 andreturn to the field and armature windings l5 and 17 via connecting line16 in a so-called free wheeling current action. The larger theinductance associated with d.c. motor 4 the longer the time thisgenerated current will continue to flow. Accordingly, absent a transientsuppression network as provided in the present invention, when one ofthe SCRs is again triggered into conduction there exists a momentaryshort between a.c. line input terminals 2 and 3 and a large voltagetransient therebetween is produced. For example, assume that abouthalfway during the positive half cycle of the a.c. input signal SCR 9 istriggered into conduction. This means that during approximately one halfthe positive half cycle of the a.c. signal SCR 9 is renderednonconductive and current flows through diodes 6 and 7 in a mannerpreviously discussed. Now when SCR 8 is suddenly triggered intoconduction a momentary short exists between a.c. input terminals 2 and 3via diode 6 and SCR rectifier 8 producing a large current transient.This results in a large transient current which will flow between theinput terminals 2 and 3 until diode 6 recovers and commences to blockthe current flow. When diode 6 does recover, a large voltage transientwill appear on input terminal 2 due to the distributed line circuitinductance.

Transient suppression network 19 of the present invention is designed toreduce such large transients between a.c. input terminals 2 and 3 due tothe switching on and off of SCR rectifiers 8 and 9. A diode 21 is poledto be rendered conductive by the induced armature e.m.f. as the magneticfields associated with the armature and field windings and 17 collapse.This means that inductor 22 and diode 21 provide an effective shunt pathfor any generated motor current during the free wheeling interval suchthat the current is prevented from flowing through diodes 6 and 7. As aresult, diodes 6 and 7 have sufficient time to recover during the offtimes of SCRs 8 and 9 t3 effecs and 3 5 tively open the circuit betweeninput terminal when one of the SCRs is gated on during the succeedinghalf cycle. For example, if line input terminal 2 is positive at theinstant that the SCRs are triggered, the current path will be fromterminal 2 through diode 6, line 16, diode 21, inductor 22, SCR 9 backto line terminal 3. lnductor 22 will limit the rate of rise of thetransient current which flows until diode rectifier 21 recovers at whichtime the current will cease to flow through diode 21, but diode nowprovides a path for the current flowing in inductor 22. Thus, it may beseen that the current transient is limited by inductor 22, and thevoltage transient by diode 20.

Numerous modifications and departures from the specific apparatusdescribed herein may be made by those skilled in the art withoutdeparting from the inventive concept of the invention. For instance, anynumber of like poled diodes may be employed in the voltage transientsuppression network and other gate controlled full wave bridge rectifierarrangements may be utilized. Further, the invention may be employedwith any type of inductive load including either series wound or shuntwound d.c. motors. Accordingly, the invention is to be construed aslimited only by the spirit and scope of the appended claims.

What is claimed is:

1. A control circuit in a d.c. system having an inductive load andadapted to be excited by an a.c. signal source comprising:

gate controlled full wave rectifier means connected to the a.c. signalsource for providing an energizing current;

means for coupling said d.c. energizing current to said inductive load;

means connected to said rectifier means for controlling the opening andclosing of the gate controlled rectifier means to vary the magnitude ofthe d.c. energizing current supplied to the inductive load; and

a transient suppression network connected in parallel with the inductiveload for reducing signal transients arising during the switching of saidgate controlled rectifier means comprising a plurality of seriesconnected, like poled, unidirectional current conducting elements and aninductive impedance means connected in parallel with at least one ofsaid unidirectional current conducting elements.

2. A transient suppression network as defined in claim 1 wherein saidunidirectional current conducting elements comprise diodes.

3. A control circuit as defined in claim 2 wherein said inductiveimpedance means comprises an inductor connected in parallel with one ofsaid diodes.

4. A control circuit as defined in claim 3 comprising in addition aserially connected resistor and capacitor connected in parallel withsaid inductive load.

1. A control circuit in a d.c. system having an inductive load andadapted to be excited by an a.c. signal source comprising: gatecontrolled full wave rectifier means connected to the a.c. signal sourcefor providing an energizing current; means for coupling said d.c.energizing current to said inductive load; means connected to saidrectifier means for controlling the opening and closing of the gatecontrolled rectifier means to vary the magnitude of the d.c. energizingcurrent supplied to the inductive load; and a transient suppressionnetwork connected in parallel with the inductive load for reducingsignal transients arising during the switching of said gate controlledrectifier means comprising a plurality of series connected, like poled,unidirectional current conducting elements and an inductive impedancemeans connected in parallel with at least one of said unidirectionalcurrent conducting elements.
 2. A transient suppression network asdefined in claim 1 wherein said unidirectional current conductingelements comprise diodes.
 3. A control circuit as defined in claim 2wherein said inductive impedance means comprises an inductor connectedin parallel with one of said diodes.
 4. A control circuit as defined inclaim 3 comprising in addition a serially connected resistor andcapacitor connected in parallel with said inductive load.